Clean-up crews have yet to reach the site of the pipeline break nearly a week after the rupture, which leaked 42,000 US gallons (159,000 litres) of oil into the Yellowstone, one of the last undammed rivers left in America.

State officials in Montana criticised oil company executives for offering conflicting accounts of the pipeline breach and its safety record.

SkyTruth, which came to prominence last year for satellite maps tracking the BP oil spill in the Gulf of Mexico, has also questioned Exxon's initial estimates of the size of the leak. SkyTruth's founder, John Amos, said his calculations suggested a leak of 63,000 US gallons, or nearly half again as much as Exxon's estimate of about 42,000 US gallons.

Environmental organisations, meanwhile, accused federal government regulators of failing to ensure safe operation of the pipeline until it was too late.

"We don't need regulators to tell us that a pipeline gushing oil into our rivers is not operating safely. We need them to create rules and standards that ensure pipelines don't do that in the first place and we don't seem to have that," said Anthony Swift, energy campaigner at the Natural Resources Defence Council.

The oil company and federal government officials believe that severe flooding eroded the riverbed in which the pipeline was buried, exposing the structure to damage. Ken Olson, the mayor of the nearby town of Laurel, Montana, said the Exxon crew were at work two weeks ago trying to protect the pipeline. He said he saw crews building a berm around a valve.

"We've experienced erosion last year, and again this year we saw even more. The amount of erosion we experienced this year I would consider, as an amateur, to be a 100-year event. I never saw anything like it," Olson said.

The record erosion has turned the focus towards the depth of the pipeline below the riverbed.

In filings with the pipeline safety authority last December, Exxon claimed that the pipeline was at least 5 feet (1.5 metres) beneath the riverbed. The pipeline authority had faulted the oil company for a series of other probable violations in July 2010.

After a temporary shutdown of the pipeline last May, following safety concerns being raised by local officials, Exxon reported on 1 June the line was at a depth of 12 feet. However, ExxonMobil Pipeline Company president Gary Pruessing said on Wednesday he could not verify that figure.

It was the third discrepancy in Exxon's account of the pipeline. The oil company had initially claimed that it took 30 minutes to shut off the pipeline, when it fact it took 56 minutes.

The company was also forced to acknowledge that oil from the ruptured pipeline had caused far wider damage than its initial claims of a 10-mile stretch of the river. The pipeline authority said aerial surveillance had detected oil as far as 240 miles away from the breach.

A spokesman for the pipeline authority refused to confirm Exxon's claims to have buried the pipeline at the greater depth of 12ft. He also gave no indication that the safety authority had directed Exxon to increase the amount of earth shielding the pipeline, despite forecasts of an unusually heavy flood season.

"Exxon made two relatively reckless move. One was building a pipeline that shallow in a flood prone river. The second was to restart the pipeline in May despite heavy flooding," said Alex Swift, the pipeline safety campaigner for the Natural Resources Defence Council. "But again a key issue here is that it was allowed to do that by the regulators."